BOOKS

The only book that I have read so far on ESP8266 is this one from Neil Kolban. You can download it as a PDF and make a donation if you wish.

Kolban's Book on ESP8266

This book is well organised and the contents page is detailed.
The 2016 edition, which was in more or less note format with many gaps, has been superseded by a 2018 edition that fills in those gaps, adds more
detail and has nearly triple the number
of pages.